Terpenes representative structures

Although the term terpene once referred only to hydrocarbons current usage includes functionally substituted derivatives as well grouped together under the general term isoprenoids Figure 26 6 (page 1086) presents the structural formulas for a number of representative examples The isoprene units m some of these are relatively easy to identify The three isoprene units m the sesquiterpene farnesol, for example are mdi cated as follows m color They are joined m a head to tail fashion... 

A considerable number of mycotoxins that show high toxicity to vertebrates and/ or invertebrates are produced by organisms associated with crop plants (Flannigan 1991). There are many known cases of human poisoning caused by such compounds. There are three broad categories of mycotoxins represented here, based on the structures of the intermediates from which these secondary metabolites are derived. They are (1) compounds derived from polyketides, (2) terpenes derived from mevalonic acid, and (3) cyclic peptides and derivatives thereof. 

Some representative Claisen rearrangements are shown in Scheme 6.14. Entry 1 illustrates the application of the Claisen rearrangement in the introduction of a substituent at the junction of two six-membered rings. Introduction of a substituent at this type of position is frequently necessary in the synthesis of steroids and terpenes. In Entry 2, formation and rearrangement of a 2-propenyl ether leads to formation of a methyl ketone. Entry 3 illustrates the use of 3-methoxyisoprene to form the allylic ether. The rearrangement of this type of ether leads to introduction of isoprene structural units into the reaction product. Entry 4 involves an allylic ether prepared by O-alkylation of a (3-keto enolate. Entry 5 was used in the course of synthesis of a diterpene lactone. Entry 6 is a case in which PdCl2 catalyzes both the formation and rearrangement of the reactant. 

While the pheromones of Tenebrio and Tribolium originate from a mixed biosynthesis, those produced by males of the broad horned flour beetle, Gnatho-cerus cornutus, represent true terpenes. Initially, the configuration of this new pheromone had been erroneously proposed to be (IR,4R,5S) a-acoradiene [325] however, independent syntheses of pure stereoisomers [326,327] proved the correct structure to show (lS,4A,5A)-configuration 165. The scope of the synthesis is shown by Mori (see chapter by Mori in volume 1 and [15]). A minor component of the G. cornutus was reported to be a-cedren-14-al 166 [328]. 

A wide variety of cyclic terpene hydrocarbons are known and, as multiples of CsH8, these have fewer double bonds than the open-chain terpenes. Because it is time consuming to show all the carbon and hydrogen atoms of such substances, the structures often are drawn in a convenient shorthand notation wherein the carbon-carbon bonds are represented by lines, carbon atoms being understood at the junctions or the ends of lines. By this notation, myrcene can be represented by formulas such as the following ... 

Similar to other natural products, phytotoxins are classified on the basis of their structural types considering their biosynthetic pathways. Among various phytotoxin families, polyketides including aromatic and reduced polyketides, peptides including diketopiperazines, and terpenes are most frequently described in the literature. In this section, representative phytotoxins that are well-characterized biosynthetically and physiologically will be introduced. 

Terpenes are composed of isoprenyl (C-,) units and are conveniently grouped as monoter-penes (skeletal basis CI0 = 2X C-,), sesquiterpenes (G13 = 3X C3), diterpenes (C20 = 4X C-,), triterpenes (C3o = 6X C-,) and tetraterpenes (C40 = 8X G-j. The structures of some representative terpenes are shown in the Appendix (Section 3). Terpenes ultimately derive biosynthetically from acetate (C2) via the activated acetyl thioester (CH3—CO—S—X) acetyl-coenzyme A (acetylCoA CH3-CO-S-C0A) as outlined below (enzymes catalysing key steps being indicated in parentheses). 

If as above we simply represent alicyclic rings sharing two Gs by a vertical line, then we can represent the basic tetracyclic structure of lanosterol as G61G61 G6 C5 (noting that there are two double bonds and various alkyl substituents and also a 3-hydroxyl on the first of the alicyclic rings). Many subsequent reactions yield cholesterol, a major triterpene membrane component that modifies the fluidity of animal cell membranes and is a precursor for synthesis of animal bile acids (fat solubilizing amphipathic detergents) plant triterpenes and steroid hormones such as the corticosteroids cortisol and cortisone, the mineralocorticoid aldosterone and the sex hormones testosterone and 17-(3-oestradiol. The structure and bioactivity of the plant terpenes is sketched below. 

Later in 1997, Ondeyka et a/.83 reported the isolation and identification of the structure of nodulisporic acid (27) using micro-NMR probe technology. Nod-ulisporic acid is a novel insecticide from Nodulisporium sp. and was the first representative of a new class of indole terpenes to be identified. The authors used a combination of -detected homo- and heteronuclear 2D-NMR experiments to assemble the structure in conjunction with a series of three INADEQUATE, each 5 days long, optimized from 40 to 60 Hz that employed a 26 mg sample of 27. The INADEQUATE data were subjected to computer analysis,84 86 which identified 32 of 47 possible 13C-13C connectivities. 

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